Fix accounting of dropped prefetched GetPage requests

[MMeent]

Apparently, we failed to do this bookkeeping in quite a few places...

Problem

Fixes https://github.com/neondatabase/cloud/issues/22364

Summary of changes

Add accounting of dropped requests. Note that this includes prefetches dropped due to things like "PS connection dropped unexpectedly" or "prefetch queue is already full", but not (yet?) "dropped due to backend shutdown".

[github-actions]

7227 tests run: 6875 passed, 0 failed, 352 skipped (full report)

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Flaky tests (3)

Postgres 17

• test_physical_replication_config_mismatch_too_many_known_xids: debug-x86-64

Postgres 16

• test_physical_replication_config_mismatch_too_many_known_xids: release-arm64

Postgres 14

• test_lr_with_slow_safekeeper: release-arm64

Code coverage* (full report)

• functions: 31.2% (8411 of 26948 functions)
• lines: 48.0% (66762 of 139204 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
2b51171 at 2025-01-03T21:18:25.876Z :recycle:}